[patch] Fix BZ 18985 out of bounds access in strftime

[Paul Eggert]

Paul Pluzhnikov wrote:
> +  /* Do range checks first.  BZ #18985  */
> +  if (tp->tm_sec < 0 || tp->tm_sec > 60
> +      || tp->tm_min < 0 || tp->tm_min > 59
> +      || tp->tm_hour < 0 || tp->tm_hour > 23
> +      || tp->tm_mday < 0 || tp->tm_mday > 31
> +      || tp->tm_mon < 0 || tp->tm_mon > 11
> +      || tp->tm_wday < 0 || tp->tm_wday > 6
> +      || tp->tm_yday < 0 || tp->tm_yday > 365
> +      || tp->tm_isdst < 0 || tp->tm_isdst > 1)
> +    {
> +      __set_errno (EINVAL);
> +      return 0;
> +    }

That doesn't look right.  If the values are out of range, C11 and POSIX say 
strftime is supposed to store an unspecified string, not return 0.

tzcode strftime stores "?" for out-of-range tm_mon and tm_wday, and looks only 
at the sign of tm_isdst; this conforms to the standard.  FreeBSD strftime is 
similar.  glibc could do the same; that should be both easy and compatible.